1. Field of the Invention
This invention relates to thermal inkjet printing and more particularly to the selection for activation of heater resistors within an inkjet printhead to expel ink from nozzles corresponding to the heater resistors.
While the present invention is described herein with reference to illustrative embodiments for particular applications, it should be understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional modifications, applications, and embodiments within the scope thereof and additional fields in which the present invention would be of significant utility.
2. Description of the Related Art
A major goal in an inkjet printer is to maximize print quality and speed while minimizing cost. To achieve this, more ink drop spray nozzles must be added to the pen while minimizing the circuit area needed. A major factor in chip area is the area of the interconnect pads which connect the die to the pen tape automated bonding (TAB) circuit. Decreasing the amount of interconnect pads on the chip not only reduces die area and cost but also tape automated bonding (TAB) circuit area as well as drive electronics in the product. The integrated drive head (IDH) is a means of reducing the printhead interconnect pads through the use of switching transistors formed on an integrated circuit substrate. The basic circuit consists of a heater resistor in series with a field effect transistor (FET) which controls the current through the resistor. By allowing current to flow through this resistor, power is dissipated in the resistor heating the ink and ejecting it through a nozzle. In the pen there are hundreds of these circuits.
A conventional printhead has 200 nozzles and is designed with 8 groups of 25 pairs each consisting of a heater resistor in series with a field effect transistor (FET). Each group has 1 primitive select, 1 ground and 25 address lines which are shared between all groups. Therefore for 8 groups of 25 pairs, there are a total of 8+8+25=41 interconnect pads required. To implement a 300 nozzle printhead, it is necessary to increase the number of groups to 12 resulting in 12+12+25=49 interconnect pads to the printhead. FIG. 1 is an illustrative schematic diagram of a conventional two dimensional address control for a 300 nozzle integrated printhead having 12 primitive selects .times. 25 address selects. The grounds are not used for addressing and are always tied to a common ground. To turn on a particular transistor, one drives high the associated primitive select and address line select.
For purposes of this present patent document only, the phrase "individual nozzle driver" means the aggregate of one or more electrical components associated closely with each individual nozzle.
For example in FIG. 1 at a representative unit position near lower left in the circuit array the "individual nozzle driver" includes one heater resistor 22 and one transistor 20. For purposes of clarifying the definition only: if ground leads were used for addressing (not part of the prior art shown), the "individual nozzle driver" could in purest principle include only one electrical component, namely a resistor.
If desired, the representative individual nozzle driver of FIG. 1 may also be deemed to include the intimately associated interconnect terminals 44j, 42j. In a sense these terminals are electrical components.
As another clarifying example, consider a piezoelectric inkjet system such as that described in IBM Technical Disclosure Bulletin, Vol. 25, No. 11A (April 1983), by G. L. Ream, entitled "Multiplex Drivers for a Drop-on-Demand Print Head". Each unit assemblage of a DOD crystal, resistor and diode shown in FIGS. 2 and 3 of that paper is an "individual nozzle driver".
(This definition may require equivalent interpretation, to the extent that components in some types of inkjet systems may be shared by a small number of nozzles, e.g. say two or three nozzles, within the printhead.)
The conventional two dimensional multiplexing scheme for printheads has the disadvantages that as the print quality and the number of nozzles increases, the number of interconnect pads to the printhead increases, which increases the printhead cost and both the die and tape automated bonding (TAB) area. This in turn increases the number and cost of the drive electronics and printer flex. In addition, more interconnect pads reduce product reliability and reduce the area available for additional circuitry for electro-static discharge (ESD) protection.
Accordingly, there is a need in the art for a system and/or technique for reducing the number of interconnect pads for a high density integrated printhead to minimize costs and increase the reliability thereof.